One of the most important quality requirements of all paper and board grades is uniformity of the structure both on the micro scale and on the macro scale. The structure of paper, in particular of printing paper, must also be symmetric. The good printing properties required from printing paper connote good smoothness, evenness, and certain absorption properties of both faces of the paper. The properties of paper, in particular the symmetry of density, is affected to a considerable extent by the operation of the press section of the paper machine, which operation has also a decisive significance for the uniformity of the profiles of the paper in the cross direction and in the machine direction.
Increased running speeds of paper machines create new problems to be solved, which problems are mostly related to the runnability of the machine. Currently, running speeds of paper machines are up to about 1500 meters per minute. At these running speeds, so-called closed press sections, which comprise a compact combination of press rolls arranged around a smooth-faced center roll, usually operate satisfactorily. As examples of such press sections, the assignee's Sym-Press II.TM. and Sym-Press O.TM. press sections should be mentioned.
From the point of view of energy economy, dewatering of a paper web taking place by pressing is preferable to dewatering taking place by evaporation. For this reason, attempts should be made to remove a maximum amount of water out of the paper web by pressing in order that the proportion of water to be removed by evaporation could be made as small as possible. However, increased running speeds of paper machines create new problems expressly for the dewatering taking place by pressing because the press impulses in roll presses (which occur in dewatering by pressing) cannot be increased sufficiently, above all because at high running speeds, the nip times remain inadequately short. On the other hand, the peak pressure of pressing cannot be increased beyond a certain limit without destruction of the structure of the web.
With increasing running speeds of paper machines, the problems of runnability of a paper machine are also manifested with further emphasis because a web with a high water content and low strength does not withstand an excessively high and sudden impulse of compression pressure or the dynamic forces produced by high speeds and changes in direction. Rather, web breaks and other disturbance of operation arise which result in standstills of the paper machine.
A further drawback of conventional prior art press sections is the need of suction energy of suction rolls, which are commonly used in such press sections, and the additional drawback of the level of noise arising from the suction rolls. Further, the suction rolls, with their perforated mantles, inside suction boxes and other suction systems, are expensive components that require repeated servicing.
With respect to the prior art related to the present invention, reference is made to the assignee's Finnish Patent Application 905798 (equivalent to EP publication 0 487 483 A1 and U.S. patent application Ser. No. 07/795,043, the specification of which is hereby incorporated by reference herein, as well as U.S. patent application Ser. No. 08/025,851, now U.S. Pat. No. 5,389,205, which is a continuation-in-part of the '043 application) which describes a method which comprises a combination of the following steps: transferring the paper web from a forming wire onto a wire in the dryer section while constantly on support of a fabric that receives water, a transfer fabric, or of any other, corresponding transfer face as a closed draw, preferably at a speed that is higher than about 25 m/s to about 30 m/s; dewatering the paper web by means of at least two successive press nips, of which nips at least one press nip is a so-called extended-nip zone, whose length in the machine direction is larger than about 100 mm, and the extended-nip zone is formed in connection with a mobile flexible press-band loop; and regulating and/or selecting the distribution of the compression pressure employed within the extended-nip press zone both in the cross direction of the web and in the machine direction so as to set or to control the different profiles of properties of the web. A device including elements for performing the above-mentioned steps is also described in the Finnish patent application.
It is a further important feature of the method and the device of FI 905798 that the paper web is not passed through the press section on only one press fabric. Rather, in order to guarantee an adequate dewatering capacity, an arrangement of fabrics is employed in which the web is transferred from the pick-up point on the first upper fabric through the first press zone, preferably an extended-nip zone, through which zone the first lower fabric runs, onto which the web is transferred after the nip zone. From the first lower fabric, the web is transferred onto the second upper fabric which carries the web into the second nip zone, which is a roll nip or preferably an extended-nip zone. After the second nip zone, the web is transferred onto the second lower fabric, which runs through the second nip zone and which carries the web on its upper face, as a closed draw, onto the drying wire or into the next, following nip zone.
With respect to the prior art closely related to the present invention, reference is made further to the assignee's Finnish Patent Application No. 935501, filed on Dec. 8, 1993 (equivalent to EP Pat. Appl. 94119255.1 and corresponding to U.S. patent application Ser. No. 08/332,861, the specification of which is hereby incorporated by reference herein). In that patent application, a press section is described in which a combination of the following characteristics has been considered inventive: the first nip in the press section is an extended-nip press having a press zone through which two opposite press fabrics that receive water are passed, so that in the first extended-nip press the dewatering takes place in two directions through both faces of the paper web; the upper press fabric in the extended-nip press is a pick-up fabric which carries the paper web from the forming wire on its lower face; at least two roll nips in the press section have been formed in connection with a smooth-faced center roll, which center roll is arranged at a level substantially higher than the level of the extended-nip press, and of which roll nips, in the first roll nip the press fabric consists of the pick-up fabric, and the second roll nip has a press fabric of its own that receives water; and, after the first extended-nip press, the running direction of the paper web has been turned at an angle a which is selected to be greater than or equal to about 45.degree..